The pericardium invests the entire heart and the cardiac portion of the great vessels, and it is reflected onto the cardiac surface as the epicardium. The sac normally contains a small amount of fluid, which provides lubrication for the continuous movement of the enclosed heart. The pericardium is not very distensible; it strongly resists a large, rapid increase in cardiac size and hence prevents sudden overdistention of the chambers of the heart. However, in congenital absence of the pericardium or after its surgical removal, cardiac function is not seriously affected. Nevertheless, with the pericardium intact, an increase in diastolic pressure in one ventricle increases the pressure and decreases the compliance of the other ventricle.
Four sounds are usually generated by the heart, but only two are ordinarily audible through a stethoscope. With electronic amplification, the less intense sounds can be detected and recorded graphically as a phonocardiogram. This means of registering faint heart sounds helps delineate the precise timing of the heart sounds in relation to other events in the cardiac cycle.
The first heart sound is initiated at the onset of ventricular systole (
Fig. 16.42 ) and reflects closure of the AV valves. It is the loudest and longest of the heart sounds, has a crescendo-decrescendo quality, and is heard best over the apical region of the heart. The tricuspid valve sounds are heard best in the fifth intercostal space just to the left of the sternum; the mitral sounds are heard best in the fifth intercostal space at the cardiac apex.
•Fig. 16.42 Thefirstandsecondheartsoundsofthephonocardio-gram(bottom tracing) areshowninregardtotheirrelationshiptotheP,R,andTwavesoftheelectrocardiogram(top tracing). 0 0.2 1.2 1.40.8 1.00.4 0.6 1.6 Time (sec) Phonocardiogram ElectrocardiogramP R T 2 1
The second heart sound, which occurs with abrupt closure of the semilunar valves (see
Fig. 16.42 ), is composed of higher frequency vibrations (higher pitch) and is of shorter duration and lower intensity than is the first heart sound. The portion of the second sound caused by closure of the pulmonic valve is heard best in the second thoracic interspace just to the left of the sternum, whereas that caused by closure of the aortic valve is heard best in the same intercostal space but to the right of the sternum. The aortic valve sound is generally louder than the pulmonic valve, but in cases of pulmonary hypertension, the reverse is true. The nature of the second heart sound changes with respiration. During expiration, a single heart sound is heard that reflects simultaneous closing of the pulmonic and aortic valves. However, during inspiration, closure of the pulmonic valve is delayed, mainly as a result of increased blood flow from an inspiration-induced increase in venous return.
With this delayed closure of the pulmonic valve, the second heart sound can be heard as two components; this is termed physiological splitting of the second heart sound.
A third heart sound is sometimes heard in children with thin chest walls or in patients with left ventricular failure. It consists of a few low-intensity, low-frequency vibrations heard best in the region of the cardiac apex. The vibrations occur in early diastole and are caused by the abrupt cessation of ventricular distention and by the deceleration of blood entering the ventricles. A fourth, or atrial, sound consists of a few low-frequency oscillations. This sound is occasionally heard in individuals with normal hearts. It is caused by the oscillation of blood and cardiac chambers as a result of atrial contraction.
The Cardiac Cycle
The phase between the start of ventricular systole and opening of the semilunar valves (when ventricular pressure rises abruptly) is called the isovolumic (literally, “same volume”) contraction period. This term is appropriate because ventricular volume remains constant during this brief period (see